1. Field of the Invention
The present invention relates to a hydraulic servo-type material testing apparatus.
2. Description of the Related Art
In a hydraulic servo-type material testing apparatus, generally, a load mechanism for applying a load to a test piece is driven by a hydraulic actuator. A target value concerning a physical value selected as a controlled variable is supplied from a waveform generator or the like. At the same time, the load according to the target value is applied to the test piece by feeding back a detected value of that controlled variable. For example, the physical value selected as the controlled variable is a displacement of the load mechanism, the load applied to the test piece in consequence of that displacement, or the like.
In the hydraulic servo-type material testing apparatus having such a feedback loop, so-called PID control is conventionally performed in which a deviation obtained by feeding back a detected value to a target value is subjected to proportional-integration-derivative (PID) operations so as to obtain a manipulated variable. To show an example of the configuration of its control system, as shown in FIG. 3, the arrangement provided is such that a detected value (observed variable) obtained by detecting a physical quantity selected as a controlled variable z, such as the displacement of the load mechanism, the load applied to the test piece or the like is fed back to a target value r outputted from such as a waveform generator 31. Then, its deviation e is introduced to a PID adjuster 33 to perform proportional-integration-derivative operations, thereby obtaining a manipulated variable u for changing a valve opening of a servo valve 34, so as to control the driving of a hydraulic actuator 35 for the load mechanism. It should be noted that w denotes a disturbance. Specifically, variations in the hydraulic pressure, the wear of a seal portion of the hydraulic actuator, and the like are conceivable as the disturbances.
This control system is shown more specifically in the block diagram of FIG. 4.
In FIG. 4, reference numeral 1 denotes a proportional element; Kp, a proportional gain; Tr, integral time; 1/s, an integral element; TD, derivative time; D(s), a derivative element; and P(s), a transfer characteristic of a controlled system.
Incidentally, with the conventional hydraulic servo-type material testing apparatus adopting the above-described PID control, there is a problem in that it is difficult to optimize both the target value response and the disturbance response.
Namely, if, as shown in FIG. 5A, adjustment is made in which the response to the target value is optimized, the response to a disturbance becomes large, as shown in FIG. 5B. On the other hand, if, as shown in FIG. 6B, optimum adjustment is made so that the response to the disturbance becomes small, the response to the target value becomes inordinately large, as shown in FIG. 6A.
In addition, in the conventional hydraulic servo-type material testing apparatus adopting the PID control, the integral element 1/s is included in the transfer characteristic P(s) of a hydraulic drive system, which is the controlled system, as well as the integral element 1/s in an adjusting unit. Therefore, the term of 1/s2 is included in the transfer functions of inputs and outputs. Hence, there is a problem in that control becomes unstable, and in an extreme case there is a possibility of the occurrence of hunting.
The invention has been devised in view of the above-described circumstances, and its object is to provide a hydraulic servo-type material testing apparatus which is capable of optimizing both the target value response and the disturbance response, and of stabilizing the control.
To attain the above object, in accordance with the invention, there is provided a hydraulic servo-type material testing apparatus having a feedback loop for controlling a hydraulically operated-type load mechanism which applies a load to a material, the apparatus comprising an adjusting unit in the feedback loop, the adjusting unit having a first portion for effecting a proportional action and a derivative action with respect to a target value and a second portion for effecting a proportional action and a derivative action with respect to a detected value.
In the hydraulic servo-type material testing apparatus, it is preferable that parameters for the proportional actions and derivative actions of the first and second portions are independently set.
Further, it is preferable that the parameters are a weight to be applied to the first portion and a weight to be applied to the second portion.
In the invention, the fact that the first and second portions of the adjusting unit in the feedback loop both effect proportional action and derivative action means that proportional elements and derivative elements function in the first and second portions of the adjusting unit. Accordingly, as for integral elements, the invention includes both a configuration in which the integral elements are absent in the adjusting unit and a configuration in which even if they are present, the integral time is set to be long to such an extent that they substantially do not function.
In the invention, the adjusting unit in the feedback loop effects proportional action and derivative action, and does not effect integral action. Namely, by substantially adopting PD control, the term of 1/s2 is eliminated from the transfer functions of inputs and outputs of the system. At the same time, the adjusting unit consists of a first portion acting with respect to a target value and a second portion acting with respect to a detected value. By setting parameters for the proportional actions and the derivative actions of the first and second portions independently, especially, applying weights to the first and second portions independently, optimization adjustment of both the target value response and the disturbance response is realized.
Namely, in the hydraulic servo-type material testing apparatus, since 1/s is included in the transfer characteristics of the hydraulically-operated load mechanism, which is the controlled system, if the integral element 1/s were made to function in the adjusting unit, the term of 1/s2would be included in the transfer functions of inputs and outputs. In the invention, however, since the integral elements are not made to function in the adjusting unit, the term of 1/s2 is not included in the transfer functions of inputs and outputs, thereby making it possible to eliminate a factor for instability of control.
In addition, the first portion acting with respect to the target value and the second portion acting with respect to the detected value are provided separately in the adjusting unit, and arbitrary parameters, especially, weights are respectively applied to these portions. The smaller the weight for the first portion is, the sharper changes in outputs can be suppressed even if there occur sudden changes in the inputs of the target value, thereby making it possible to suppress the occurrence of overshoots. Nevertheless, the rise of outputs becomes slow. On the other hand, if the proportional gain which is equally applied to both of the first and second portions is made large, the feedback is reinforced and disturbances are controlled more powerfully. However, if the proportional gain is made excessively large, the system becomes oscillatory and becomes unstable. For these reasons, by performing a simulation in advance, the weight, which is to be applied to the first portion acting with respect to the target value, of the two portions of the adjusting unit, and the value of the proportional gain are appropriately adjusted, and the value for controlling the disturbance is determined and set in a state that sharp changes will not occur in the outputs and the system does not become oscillatory. It is thereby possible to realize a control system which is capable of controlling disturbances powerfully and in which sharp changes do not occur in the outputs. Consequently, it is possible to obtain a control system which is able to simultaneously obtain the target value response such as the one shown in FIG. 5A and the disturbance response such as the one shown in FIG. 6B.